Disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording data sectors and servo sectors. The servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo control system to control the position and velocity of the actuator arm as it seeks from track to track.
FIG. 1 shows a prior art disk format 2 as comprising a number of servo tracks 4 defined by servo sectors 60-6N recorded around the circumference of each servo track. Each servo sector 6i comprises a preamble 8 for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark 10 for storing a special pattern used to symbol-synchronize to a servo data field 12. The servo data field 12 stores coarse head positioning information, such as a servo track address, used to position the head over a target data track during a seek operation. Each servo sector 4, further comprises groups of servo bursts 14 (A,B,C,D in the example shown), which are recorded with precise intervals and offsets relative to the track centerlines. The servo bursts 14 provide fine head position information used for centerline tracking while accessing a data track during write/read operations.
Control circuitry is typically mounted on a printed circuit board (PCB) that is fastened to the base of the disk drive. The head is connected to the control circuitry through a flex circuit fastened on one side to the PCB and fastened on the other side to the side of the actuator arm. The flex circuit may induce a bias force on the actuator arm in addition to other bias forces, such as bearing friction in the actuator arm pivot bearing, and windage affecting the actuator arm as the disk rotates. If the bias forces on the actuator arm are not compensated correctly, it may cause the settle mode at the end of a seek to fail. This is illustrated in the prior art flow diagram of FIG. 2A which is understood with reference to FIG. 2B. During an access operation (write or read), the control circuitry seeks the head toward a target track (block 16). When the head nears the target track, the control circuitry transitions into a settle mode (block 18) and attempts to settle the head onto the target track (block 20) as illustrated in FIG. 2B. If the seek operation successfully settles the head onto the target track, the seek operation exits normally (block 22). However, various factors may prevent the successful completion of the normal seek operation. For example, if the bias forces applied to the actuator arm are not accurately compensated, the normal seek operation may be unable to overcome these bias forces resulting in a time-out (block 24) as illustrated in FIG. 2B. When the normal seek operation times out (after one or more retry operations), the control circuitry will typically report an error to the host (block 26).